The description relates to liquid crystal displays.
Liquid crystal displays (LCDs) can be used in, e.g., portable devices, computers displays, and high definition televisions. A liquid crystal display can have a liquid crystal layer and two crossed linear polarizers for modulating light using an electro-optic effect. An external voltage applied to the liquid crystal layer changes the orientations of the liquid crystal molecules and the optical phase retardation of the liquid crystal layer, thereby changing the amount of light that passes the crossed linear polarizers. Each pixel of the display can show a range of gray scale levels depending on the voltage applied to the liquid crystal layer. Color filters can be used to filter light to generate color.
The optical characteristics of a liquid crystal display are affected by the molecular arrangements of liquid crystal molecules when no voltage is applied (referred to as the “initial state”) and when voltages are applied (referred to as the “operation state”) to the liquid crystal layer. The initial arrangement of the liquid crystal molecules can be determined by, e.g., surface boundary conditions. The liquid crystal layer is between two substrates, and the surface boundary conditions can be controlled by alignment layers attached to the substrates. Each alignment layer can be, e.g., a thin film of organic (e.g., polymer) or inorganic material(s).
The liquid crystal molecules are initially aligned perpendicular or parallel to the surface of the alignment layer with a small inclination (pretilt) along a certain direction. The direction of inclination or tilt defines the molecular reorientation direction in the operation state. The amount of the inclination is called a pretilt angle. The surface structure of the alignment layer that defines the surface pretilt angle can be obtained by buffing the organic alignment layer, exposing polarized or unpolarized light from an inclined direction on the organic alignment layer, or inclined deposition of an inorganic alignment layer. When a voltage is applied to the liquid crystal layer in the operation state, the applied electric field exerts a torque on the liquid crystal molecules due to dielectric anisotropy of the molecules. The initial structure of the liquid crystal layer together with the molecular reorientation scheme defines a liquid crystal mode. Different liquid crystal modes can be used in different applications.
For example, displays having different sizes can use different liquid crystal modes due to considerations in device fabrication complexity, manufacturing costs, and system performances. For small and mid-sized screens (e.g., those used in mobile phones and computer monitors), a twisted nematic (TN) mode can be used. TN displays are described in “Voltage-Dependent Optical Activity of a Twisted Nematic Liquid Crystal,” by M. Schadt et al., Applied Physics Letters, Vol. 18, p. 127 (1971). TN displays can be reliable to operate and simple to manufacture. In a TN display, two substrates are provided with alignment layers that align the liquid crystal molecules parallel to the substrate surfaces in the initial state. The top and the bottom alignment layers are rubbed along orthogonal directions. Due to this boundary condition, the liquid crystal layer has a twisted structure when no voltage is applied to the liquid crystal layer. This twisted structure changes the polarization state of light that passes the liquid crystal layer due to birefringence and wave guiding effects. The wave guiding effect provides a high transmittance efficiency at the bright state with low color dispersion, compared to other liquid crystal modes that uses only the birefringence effect for the bright state.
The term “twisted structure” refers to a condition of the liquid crystal layer in which the orientations of the directors of the liquid crystal molecules are different at different positions along a vertical direction. The twisted structure can be similar to a helix. A clockwise twist direction means that the liquid crystal molecules have orientations that rotate in the clockwise direction as the liquid crystal molecules move from positions closer to the back side of the display to positions closer to the front side of the display (similar to a left handed helix). A counter clockwise twist direction means that the liquid crystal molecules have orientations that rotate in the counter clockwise direction as the liquid crystal molecules from positions closer to the back side of the display to positions closer to the front side of the display (similar to a right handed helix).
The TN display can be switched to a dark state by applying an operation voltage to the liquid crystal layer, causing the liquid crystal molecules to be oriented perpendicular to the substrate surface. In the dark state, there can be light leakage caused by optical retardation at the surface regions of the liquid crystal layer because the liquid crystal molecules near the surface regions are not switched perpendicular to the substrate due to the binding force of the alignment layers.
In another liquid crystal mode, referred to as the vertical alignment (VA) mode, the liquid crystal molecules are initially aligned in the vertical direction (i.e., perpendicular to the surface of the substrates). There are two types of VA modes. The first type uses a birefringence effect to control brightness, and is referred to as the electrically controlled birefringence (ECB) VA mode. See “Deformation of Nematic Liquid Crystals with Vertical Orientation in Electrical Fields,” by M. F. Schiekel et al., Applied Physics Letters, Vol. 19, p. 391 (1971). The ECB VA mode uses alignment layers that align the liquid crystal molecules perpendicular to the substrate surface. The rubbing directions of the top and bottom alignment layers are opposite to each other. To achieve a high brightness, the optic axes of the top and bottom polarizers have transmission axes oriented at 45 degrees relative to the rubbing directions of the alignment layers.
Note that the terms “vertical” and “horizontal” are used to describe the relative orientations of various components of the display. The components can have different orientations.
A second type of VA mode, referred to as a “chiral homeotropic mode” or a “homeotropic-to-twisted planar switching mode,” has the advantages of ECB VA mode (e.g., high contrast image) and TN mode (e.g., high brightness and low color dispersion). See “Novel electro-optic effect associated with a homeotropic to twisted-planar transition in nematic liquid crystals,” Seong-Woo Suh et al., Applied Physics Letters, 68, p. 2819 (1996) and “Chiral-homeotropic liquid crystal cells for high contrast and low voltage displays,” by Shin-Tson Wu et al., Journal of Applied Physics, 82, p. 4795 (1997). The chiral homeotropic mode LCD can use a negative dielectric anisotropy liquid crystal material mixed with a small amount of chiral material.
In a chiral homeotropic mode LCD, the liquid crystal layer is sandwiched between two glass substrates that are coated with a thin layer of transparent and conductive electrode (e.g., indium tin oxide) and subsequently over-coated with a thin organic (e.g., polyimide) or inorganic (e.g., SiO2) alignment layer. The alignment layer can align the liquid crystal molecules perpendicular to the substrate surfaces in the initial state. When a voltage is applied to the liquid crystal layer, the chiral material introduces a twisted structure in the liquid crystal layer.
The tilt direction of the alignment layers on the bottom and top substrates can be different. The angle between the two tilt directions can be, e.g., 90 degrees. The different tilt directions introduce a twisted structure in the liquid crystal layer when a voltage is applied to the liquid crystal layer. The tilt directions of the alignment layers are configured to cause the liquid crystal molecules to form a twisted structure in the liquid crystal layer, in which the twist direction of the twisted structure is the same as the twist direction caused by the chiral material.
For example, if the twisted structure caused by the chiral material has a clockwise twist direction, then the tilt directions of the alignment layers are configured to cause the liquid crystal molecules to form a twisted structure having a clockwise twist direction. Conversely, if the twisted structure caused by the chiral material has a counter clockwise twist direction, then the tilt directions of the alignment layers are configured to cause the liquid crystal molecules to form a twisted structure having a counter clockwise twist direction.
The chiral homeotropic LCD has polarizers that are crossed, i.e., have transmission axes that are oriented orthogonally. The tilt direction of one of the alignment layers is parallel to one of the transmission axes of the crossed polarizers. In the initial state, the liquid crystal molecules are aligned in the vertical direction and light does not pass the crossed polarizers, resulting in a dark image. This is similar to the situation in the ECB VA mode. In the operation state, an electric field in the vertical direction is applied to the liquid crystal layer. Because the liquid crystal molecules have negative dielectric anisotropy, the applied electric field tends to reorient the liquid crystal molecules toward the horizontal direction. Due to the effect from the different tilt directions on the alignment layers and the effect from the chiral material, the liquid crystal molecules in the bulk area form a twisted structure. The twisted structure in the bulk area of the chiral homeotropic mode LCD is similar to that of the TN mode LCD and has optical properties similar to those of the TN mode LCD.